This project will develop procedures to integrate research results from other projects in this program with existing information to evaluate the plausibility of the Fries hypothesis in the light of all available evidence. The hypothesis is that biological limitations on the length of human life are driving the lifetime distribution of "medically advanced" populations to a nearly normal distribution with a mean about 85 years and standard deviation of about 7 years. The hypothesis has been widely debated and each side has adduced a number of arguments in favor of their answer, based on evidence ranging from Hayflick's observations on the finite number of doublings in cell cultures to the comparison of various statistics calculated from historical records of survivorship. To evaluate the Fries hypothesis, procedures will be developed that will be used to: (a) explicate the structure of the different arguments bearing on the truth of the Fries hypothesis, (b) for each of these arguments considered separately, evaluate the plausibility of the hypothesis in the light of the argument, and (c) merge these evaluations into an overall measurement of the plausibility of the hypothesis. This research has significance for the overall program project, for demographic projections, and for the conduct of scientific argumentation. For the overall program, this project will serve as a kind of integrative core. This project will contribute to demographic projection by evaluating the likelihood that life expectancy will level off at age 85 and also by identifying promising directions to pursue in obtaining future evidence bearing on trends in life expectancy. Finally, the proposed research can provide a model for a rational, systematic approach to scientific argumentation and paradigm adjudication.